Patch microstrip antennas are small, low-profile, low radar cross-section (RCS) and lightweight radiators ideal for phased array applications. In addition, a microstrip patch antenna is relatively inexpensive and easily manufactured, rugged, readily conformable to mount to an irregular shape, having a broad reception pattern, and can be adapted to receive multiple frequencies through proper configuration of the patches. Patch antenna radiating elements utilized for radar antenna arrays are inherently limited in bandwidth, scan angle and cross polarization. For example, a simple back-fed patch typically has a bandwidth of about 3% to about 7% of the operating center frequency. In addition to bandwidth limitations, a patch design must take into consideration several trade-offs that affect size, weight, the effects of cross polarization, excitation of surface waves and current density, sensitivity and transmission and reception angle, and power.
Patch antennas are cavity radiators where the excitation voltages are fed through the back of the substrate. As such, patch antennas have dominant electrical properties of capacitance and depending on the electrical attachments, degrees of inductance. These properties, among other things, affect the bandwidth. For a conventional patch antenna design the bandwidth may be derived according to the following:
Equation 1
  bandwidh  ⁢          ∝            h                        λ          0                ·                              ɛ            r                                ·                  W        L            
Where h is the substrate thickness or height, λO is the design wavelength, εr is the relative permittivity of the substrate and W and L are the width and length of the patch, respectively.
For example, from Equation 1 if εr=1, then the patch tends towards a wider bandwidth in free space and a further increase in permittivity (e.g., εr>1) allows the substrate to be reduced in all dimensions. However, increasing permittivity may in turn excite surface waves that contribute to scan blindness. Therefore, the permittivity of the substrate is a factor in determining antenna bandwidth and dimensions while also minimizing the possibility of surface wave excitation.
Increasing the thickness of the substrate also tends to increase the antenna bandwidth. Reducing the dielectric constant of the patch also increases bandwidth, but generally requires an increase in the thickness of the patch.
Increasing the dielectric substrate height for a back-fed patch antenna can increase operational bandwidth up to 25%, but will also disadvantageously increase the size of the antenna. Such approach is limited by increasing inductance of the feed probe, which limits the substrate thickness and possible bandwidth increase.
Hence, using a single capacitively coupled feed probe (in lieu of a conventional back fed probe) in conjunction with increased substrate dielectric height can yield operational bandwidths of about 25% while rectifying increasing probe inductance. Capacitive feeds tend to allow production of wide band patch antennas that counteract probe inductance. However, these antennas tend to be extremely sensitive to probe dimensions and position. Furthermore, cross polarized radiation remains a problem. Patches for a dominant mode exciting the current flow on the surface may cause a high cross polarization, with a maximum occurring at about 45° from bore sight. Using a dual probe feed 180° out-of-phase tends to reduce the high cross-polarization, but does not improve the operational bandwidth.
Alternative designs are desired.